In prior art, methods are known for preparing weakly or strongly superalkalinized sulfonates from sulfonic acids obtained by the sulfonation of different alkyl aryl hydrocarbons and from an excess of alkaline earth base.
The alkyl aryl hydrocarbons subjected to the sulfonation reaction are obtained by alkylation via the Friedel and Craft reaction of different aryl hydrocarbons, particularly aromatic, with two different types of olefin:
branched olefins obtained by the oligo-polymerization of propylene to C.sub.15 to C.sub.42 hydrocarbons, particularly the propylene tetrapolymer dimerized to a C.sub.24 olefin, and PA1 linear olefins obtained by the oligo-polymerization of ethylene to C.sub.14 to C.sub.40 hydrocarbons. PA1 (a) at least 50% and not more than 85% by weight of a mono alkyl phenyl sulfonate in which the mono alkyl substituent is a linear chain, containing between 14 and 40 carbon atoms, and the phenyl sulfonate radical of the alkaline earth metal is fixed, in a molar proportion of between 0 and 13%, preferably between 5 and 11%, and more particularly between 7 and 10%, in position 1 or 2 of the linear alkyl chain, and PA1 (b) at least 15% and not more than 50% by weight of a heavy alkyl aryl sulfonate selected from:
While it is easy to obtain a good dispersion in the medium of the alkaline earth base not fixed in the form of salt if the sulfonic acid is derived from a hydrocarbon obtained by alkylation of an aryl hydrocarbon with a branched olefin, it is difficult if the alkylation is effected with a linear olefin. It is particularly difficult for the alkylation of an aryl hydrocarbon containing at least 80 mole % of linear mono alpha olefin, due to the formation of a skin in the open air.
This poor dispersion is especially pronounced if the medium also contains a high proportion of sulfonate, that is if it corresponds, according to Standard ASTM D-2.896, to a low base No. BN (between 3 and 60), hence to a low content of free lime and the absence of carbon dioxide and carbonate.
In fact, during the alkylation reaction with benzene or another aromatic or aryl hydrocarbon, the molar proportion of the corresponding cyclic hydrocarbon group in position 1 or 2 of the initial linear olefinic chain is about 25%.
When prepared by the method described, for example in French Patent No. 2,564,830, this high proportion of alkyl aryl hydrocarbon having an aryl radical in position 1 or 2 of the linear alkyl chain results in a sulfonate that exhibits hygroscopic properties such that a superficial `skin` is formed. This `skin` makes this product unacceptable as an additive for lubricating oil.
Furthermore, the formation of this superficial skin is generally accompanied by a very low filtration rate, a high viscosity, a low incorporation of calcium, a deterioration of anti-rust performance, and an undesirable turbid appearance, or even sedimentation, when the sulfonate thus prepared is added at the rate of 10% by weight to a standard lubricating oil and stored for examination.
The Applicant has carried out chromatographic analyses to identify each of the different isomers differing by the position of the aryl radical on the carbon atom of the linear alkyl chain, and examined their respective influence on the properties of the corresponding alkyl aryl sulfonates of alkaline earth metals obtained from these different isomers.
The Applicant has thus discovered that he could overcome the aforementioned drawbacks, inasmuch as the molar proportion of aryl hydrocarbon, other than benzene, fixed to the carbon atoms situated in position 1 or 2 of the linear alkyl chain was between 0 and 13%, and preferably between 5 and 11%, and more particularly between 7 and 10%.
This discovery was the subject of a French Patent Application filed Mar. 8, 1995 under No. 95 02,709 by the Applicant.
Yet the Applicant had not succeeded in obtaining satisfactory results when the aryl hydrocarbon was benzene, because, heretofore, he had never been able to prevent the formation of the skin with the use of this aromatic hydrocarbon, even if said hydrocarbon was alkylated with a very long chain linear mono olefin on the carbon atom situated in position 1 or 2 of said chain in a molar proportion of between 0 and 13%, and preferably between 5 and and more particularly between 7 and 10%.